1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system for an internal combustion engine having an air-fuel ratio sensor arranged at a location upstream of a catalytic converter arranged in the exhaust system of the engine, which control system controls the air-fuel ratio of an air-fuel mixture supplied to the engine, based on an output from the air-fuel ratio sensor.
2. Prior Art
There is conventionally known a method of controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine having an air-fuel ratio sensor arranged upstream of a three-way catalyst arranged in the exhaust system of the engine, which carries out feedback control of the air-fuel ratio of the mixture supplied to the engine (hereinafter referred to as "the supplied air-fuel ratio"), based on an output from the air-fuel ratio sensor.
Further, an air-fuel ratio control system has been proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 63-147941, which carries out feedback control of the supplied air-fuel ratio, based on an output from an air-fuel ratio sensor arranged downstream of a catalytic converter arranged in the exhaust system of an internal combustion engine, and which can deal with deterioration of the catalytic converter by detecting a deterioration degree of the catalyst of the catalytic converter, and changing a skip amount (proportional term) and an integral term applied in the feedback control in response to the detected deterioration degree.
However, when the feedback control is carried out based on the output from the air-fuel ratio sensor upstream of the catalytic converter, it is difficult to enable the catalytic converter to exhibit its performance to a satisfactory degree, only by changing the skip amount and the integral term according to the deterioration degree of the catalyst as in the above conventional control system. Therefore, the conventional control system still requires reducing an amount of CO and HC components in exhaust gases.